Amazon seems to have recently updated the options for account holders in the UK, at least. They now offer the option to change the account to one based on the US site, where payments are in USD. Services such as WhisperCast are also available. So potentially you'll be able to switch your account too (depending on whether Amazon offer you this option).
I was offered these options after selecting the change device country option. Here's the text I see on my current UK-based account (I have several accounts already set to the US option via the amazon.co.uk site):
You are currently linked to shop on Amazon.co.uk. Click <here> to learn more about other Amazon sites you are eligible to shop on based on your country of residence.
Clicking the link causes the next text to appear on the page (nicely formatted):
Your Account > Manage Your Kindle > Manage Your Kindle Store
Your Kindle Store
To learn more about shopping for Kindle content and managing your Kindle account at another Amazon
Kindle store, please select from the available Amazon Kindle stores for your country below:
Prefer to shop in US Kindle Store, make purchases in US Dollars, read reviews on Amazon.com?
Learn about transferring your Kindle account to Amazon.com
Prefer to shop in UK Kindle Store, make purchases in Pounds Sterling, read reviews on Amazon.co.uk?
Your Kindle account is currently registered to Amazon.co.uk.
I've not investigated Active Content so I don't know whether it's available on converted accounts. Be aware that any remaining gift balance is not available to you once you convert the account.